Contribution à la compréhension de la fonctionnalisation mécanique de surface des composites à matrice thermoplastique (PEEK) destinés à l'assemblage par collage

L assemblage des matériaux composites thermoplastiques tel que le PEEK est l une des problématiques majeure de l industrie aéronautique. Actuellement, différentes techniques sont développées pour assurer l assemblage structural de ces matériaux, tels que : le soudage, le rivetage, le boulonnage et le collage. Les enjeux industriels majeurs sont principalement, à l heure actuelle, la conception des structures simplifiées au maximum afin de réduire les coûts de production et la réduction des consommations énergétiques. A cet effet, l industrie aéronautique fait fréquemment appel à l assemblage par collage en raison de nombreux avantages qu il offre (gain de poids, distribution régulière des contraintes, absence de trous) par rapport aux autres techniques existantes. Le PEEK (PolyEtherEtherKetone), est un matériau polymère semi-cristallin thermoplastique, à hautes performances. Ce matériau est souvent utilise dans l industrie aéronautique principalement renforce par des fibres de carbone ou de verre. Cependant, du fait du niveau élevé de sa résistance chimique l assemblage par collage du PEEK et de ses composites nécessitent des traitements de surfaces appropries et optimises. Or, afin d obtenir un system collé à haute performance, la problématique scientifique et technique doit être concentrée sur la jonction entre les éléments à assembler. En effet, la qualité de cette jonction est de la plus haute importance car elle doit permettre un transfert optimal des contraintes thermomécaniques lorsque l assemblage est soumis a ses conditions d'usage. Cette étude concerne donc, l amélioration des propriétés mécaniques (monotones et cycliques) de l assemblage par collage PEEK/PEEK. Dans cette optique, un traitement de surface simple de mise en œuvre est proposé. Ce traitement est le sablage, qui permet la modification topographique (morphologique) de surface. La compréhension des différents phénomènes d interaction aux interfaces intervenant dans l amélioration du comportement mécanique du joint de colle et qui s inscrit dans la triptyque : Rhéologie, Physico-chimie et topographie , est l enjeu scientifique majeur dans cette thèse. Dans un premier temps, l influence des paramètres du traitement tels que le temps de projection, la taille des particules, sur la morphologie de surface de différents matériaux à base de PEEK a été analysée, permettant ainsi d établir la corrélation entre les paramètres morphologiques et les mécanismes de modification topographique de surface intervenant pendant le traitement de surface. L un des facteurs clefs pour la compréhension des mécanismes d interaction entre l adhésif liquide et le substrat solide est la mouillabilité. L analyse du comportement au mouillage en fonction des différents paramètres du traitement a été réalisée. La mouillabilité des surfaces traitées est fortement affectée par la rugosité de surface créée après ce traitement. La relation entre les paramètres morphologiques et la mouillabilité a été discutée. Enfin, l influence des paramètres du traitement par sablage sur le comportement mécanique monotone et à long terme (essais de fatigue) sur la résistance du joint colle a été étudié à l aide d essais de cisaillement sur éprouvettes à simple recouvrement. Ceci a conduit, à la proposition de paramètres morphologiques surfaciques spécifiques pour l optimisation du comportement mécanique du joint de colle des matériaux composites à matrice PEEK.One of most problematic in the aeronautical industries is the structural joining of the high performance thermoplastic composites like PEEK composites. Actually, a lot of technologies are used for joining thermoplastic composites like welding, bolting, riveting, fastening and adhesive bonding. Due to the various advantages that characterize the adhesive bonding method, such an uniform stress distribution along the joint, weight light and cost reduction, makes this technique more desirable to join thermoplastic composites materials compared to the other joining techniques. PEEK (PolyEtherEtherKetone) is a semi crystalline thermoplastic material with high performance. This material is wildly used in aeronautical industries, principally, reinforced with carbon of glass fibres. However, its high chemical resistance makes the adhesive bonding of PEEK and its composites difficult and therefore an appropriate and optimised surface treatment is necessary. In the aim to obtain a bonded system with high performance, scientific and technical problematic should be focussed on the junction between adherents. Indeed, the quality of this junction is of utmost importance because it must allow optimum transfer of thermomechanical stresses when the assembly is subject to its terms of use. Though, at this time it is well known that thermoplastic composite materials are difficult to bond with out surface treatment. This study, therefore, relates to the improvement of mechanical properties (monotonic and cyclic) of the adhesive bonding system PEEK / PEEK. In this context, a surface treatment, easy to implement, is proposed. This surface treatment is sandblasting, which enables surface topographic (morphological) modifications. Understanding of various phenomena of interfaces interaction involved in the improvement of the mechanical behavior of the adhesive joint and is part of the triptych "Rheology, Physico chemistry and topography" is the major scientific challenge in this thesis. Initially, the influence of processing parameters such as the projection time, the particle size on surface morphology of various materials based on PEEK was analysed, thus allowing establishing the correlation between morphological parameters and modification mechanisms involved during surface treatment surface. One of the key factors for understanding the mechanisms of interaction between the liquid adhesive and the solid substrate is wettability. The analysis of the wetting behavior as a function of various parameters of the treatment was performed. The wettability of treated surfaces is strongly affected by surface roughness created after this treatment. The relationship between morphological parameters and wettability was discussed. Finally, the influence of sandblasting processing parameters on the mechanical behavior in monotoning and long term (fatigue tests) of the adhesive joint strength was studied, using single lap shear tests specimens. This has led to the proposal of specific surface morphological parameters for the optimization of the mechanical behavior of the adhesive joint of PEEK and its composites.LYON-Ecole Centrale (690812301) / SudocSudocFranceF

Electric charge trapping, residual stresses and properties of ceramics after metal/ceramics bonding

International audienceThe use of ceramic components in electrical engineering and mechanical applications is rapidly increasing. Most of these applications require the use of ceramics bonded with metal. In this paper,we have studied the role of residual stresses occurring after joining between an industrial alumina ceramic (Al2O3) and Ni-based super-alloy, on the dielectric behaviour of ceramics. The electric charging phenomenon i.e. trapping-detrapping or diffusion of electric charges is studied by Scanning Electron Microscope Mirror Effect (SEMME) coupled with the Induced Current Method (ICM). Knowing that localized trapped charges in ceramics is a source of damage, the correlation between residual stress intensity, apparent-toughness of ceramics and ability to trap charges near the interface was demonstrated: the SEMME and ICM measurements of the quantities of trapped charges near the interface, highlighted the changes in the ceramic properties related to residual stresses due to both thermo-mechanical effect and diffusion of metallic species in the ceramics, during the bonding process

Effect of thermal residual stresses on the strength for both alumina/Ni/alumina and alumina/Ni/nickel alloy bimaterials

International audienceThis paper describes some technical limitations encountered in joining ceramics-ceramics or ceramics-metals, and how, to some extent, they have been practically overcome. The effect of the residual stresses on the strength of joints fabricated between alumina-alumina or alumina and the nickel base alloy HAYNES 214TM using a solid-state bonding technique with Ni interlayer was studied. Finite element analyses (FEA) for the elastic-plastic and elastic-plastic-creep behavior have also been used to better design the joints and to predict their performance. It was found that the residual stresses caused by the thermal expansion mismatch between alumina (Al2O3) and the Ni-based superalloy (HAYNES 214TM) have severely deteriorated the joints compared to Al2O3-Al2O3 joint fabricated with the same solid-state bonding parameters. The high residual stresses zones obtained through the FEA simulation fitted well with the fractographic observations of the Al2O3/Ni/HAYNES 214TM joints. Also, in order to use the joint material as a structural material, the study about the effect of geometrical parameters has been performed. Optimal geometries have been determined

FEM CALCULATIONS AND EXPERIMENTAL DETERMINATION OF RESIDUAL STRESSES IN ALUMINA/NICKEL ALLOY JOINTS. OPTIMIZATION OF FABRICATION PARAMETERS

International audienceThe study relates to joints fabricated by solid state bonding between alumina and nickel alloy HAYNESTM214®, using an intermediate nickel metallic foil. Experimentally, damages and cracks often are observed close to the metal/ceramics interface. Consequently, the residual stresses distributions in the specimen were characterized experimentally using X-ray diffraction (XRD) and indentation techniques and predicted by Finite Element Analysis (FEA) calculations using an elastic-plastic-creep model. We demonstrate that a good correlation between FEA calculations and experimental results is obtained. Then, the effect of elaboration and geometrical parameters has been studied in order to minimize the residual stresses in alumina close to the metal-ceramics interface. However, the Al2O3/Ni/HAYNESTM214® system always leads to high residual stresses. To solve this problem, we show that the use of a multi-layer Cu/Ni/Cu joint, associated with the Direct Copper Bonding method (DCB), by pre-oxidation of copper, allows reducing significantly the tensile residual stresses in ceramics

Study of SiC-nickel alloy bonding for high temperature applications

International audienceWe have studied the formation of metal/ceramic joints by solid state bonding technique for applications at temperatures >600 ◦C. The bonding is obtained between silicon carbide (SiC) and Ni-based super-alloy (HAYNES® 214TM) via metallic foils (Ni, Ag). In some cases a thin coating on the ceramic or the alloy by the electroless JetMétalTM process has been used. Often used in brazing, nickel, when added to silicon carbide, usually give silicides. These reactions yield the "Pest Effect" ("pesting") that induces a catastrophic brittleness of this type of assembling. To minimize the reaction of these metals with silicon carbide, addition of elements limiting the "Pest Effect" on the one hand and, diffusion barriers on the other hand, have been performed. Indeed, the choice of the thin Ni0.93 B0.07 coating is based on the ability of boron of improving the mechanical properties of silicides, thus avoiding the "Pest Effect". However, we demonstrate that boron does not allow one to suppress the joint brittleness. Another new joining method employing a thin Ag coating or a Ag foil was tested. This process revealed the absence of chemical reaction at the Ag/SiC interface, thus proving the beneficial role of silver, which acts as an effective diffusion barrier for nickel beyond a certain thickness. This method has led to fabrication of joints presenting high shear resistance (>40MPa)